These are two approaches that can be taken to the generation of a laser beam within laser oscillators either the laser oscillator's active medium is excited as a whole or the active medium can be split into a large number of sections, the laser beam output of each of the said sections being then phased-locked together to produce a single beam equivalent to that emitted by the single section medium laser oscillator.
Two development avenues have resulted in the techniques necessary to generate a single laser beam by phase-locking the output beam of a large number of smaller laser beam emitting apertures, namely, fiber bundle laser arrays and arrays of semiconductor lasers. The fundamental difference between these two development avenues is the fact that the fiber laser aperture array is a cold array, involving only the transmission of the laser light through the said aperture whilst the semiconductor array aperture is a hot aperture because up to 75% of the electrical energy into said aperture is deposited as heat energy with in the said aperture, only about 25% of said electrical energy being converted to laser light within the diode array. Although phase-locking of semiconductor arrays is now well established, no reports are to hand that suggest such diode arrays have coherently phased-locked on a large scale, that is large diode arrays phased-locked in pockets across the aperture, a process that leads to a severe degrading of the structure of the emitted laser beam.
The inventor has pioneered key aspects of fiber bundle based phased-array lasers since 1963 when a team set up by the British Government consisting of microwave radar pioneers and laser physicists was stationed at The Royal Radar Establishment, Malvern, UK, to determine the avenues along which conventional radar techniques could be used to develop laser radar. One of the avenues studied was that for the transfer of microwave, phased-array radar techniques into the optical region and optical fiber bundles were assessed experimentally for this task by the inventor at the Royal Radar Establishment as early as 1963. However, these early experimental tests revealed that a helium-neon laser beam was converted into "non-laser light" as soon as it entered the fiber bundles available in those days and the development of fiber bundle based phased-array laser radars was held up until single mode optical fibers became available some seventeen years later during the late 1970's. A key process in phased-array laser radar utilizing bundles of single mode optical fibers was published in 1979 (Hughes and Ghatak, applied Optics, U.S.A., 1979).
Early phased-array laser radar patents by the inventor were classified by the US patent Office in June 1983 and remain classified. However, a commercially orientated phased-array, fiber bundle laser oscillator consisting of undoped optical fibers was patented in the United States in 1987 (U.S. Pat. No. 4,682,335 Hughes, July 1987). However, the prior art, fiber bundle based, phased-array lasers were difficult to assemble compared to the relative simplicity of the present invention which lends itself to simple, but highly effective mass production techniques.
The first of our looped, neodymlun doped fiber lasers was constructed and operated under contract from the assignee by YORK TECHNOLOGY Ltd of Southampton, UK in 1988. However, the individual looped fiber lasers in the 20 bundle system manufactured by YORK under contract to the assignee were side excited with a 830nm laser diode output coupled into the core of each of the looped laser oscillators in the bundle of said oscillators via a commercially available optical coupler manufactured by YORK TECHNOLOGY Ltd for the optical communications market. Unfortunately, such couplers are expensive and are not appropriate in the low cost, unswitched, diode excited looped, fiber laser bundler based laser oscillator of the invention. For example, when one packs the fiber bundle so that the fibers are in contact with each other, they represent a solid block of glass in most respects, in particular from the viewpoint of direct optical excitation. The fact that the fibers used to date have a 5 micron diameter doped core and an 80 micron diameter cladding does not affect the optical pumping because the volume of the fiber cores being excited is the beam as if the excitation light was coupled into each individual fiber with an array of very expensive optical couplers.
To achieve coherent phase-locking of the present invention is a much simpler process than is generally thought. For example, the length of the fiber loops in the individual fiber laser oscillators need not be the same, the critical length is that corresponding to a 360 degree phase change, that is the one corresponding to a complete laser wavelength within the fiber core. If the compacted end face of the invention is optically polished to say a twentieth wavelength and index matched to the output mirror surface also polished to a twentieth wavelength, then the effective optical path of all the loops irrespective of their individual physical lengths, will be equal to a twentieth of a wave and well suited for coherent phase-locking of the fiber end array. It should also be noted that the supermode of operation resulting in the coherent phase-locking of the array, is paralleled over the number of loops and is not seriesed over the total length of the loops. In other words, the mean length of the loops represents the fiber length over which the supermode has to be maintained. Furthermore, the broad gain curve of the doped glass fibers used means that the supermodes in different looped fibers can differ in wavelength. In other words, supermode pulling effects in individual fibers can also contribute to coherent phase-locking across the aperture of the invention.
To minimize the cost of manufacturing the invention, it is an advantage to be able to use the optical fiber manufactured worldwide for optical communications needs. These fibers generally have a core diameter of about 5 microns with a cladding diameter of about 125 microns. Such thick cladding is necessary to minimize the optical signal loss from the signal transmitting core of the fiber. Such thick cladding also protects the said fiber core from mechanical damage, a base 5 micron diameter core being extremely fragile. By design there is not transverse optical coupling between such fiber cores in an array of such fibers so that the phase-locking process has to be achieved either by reflective or refractive coupling, a combination of both and some transverse optical coupling via the index matching material be it liquid or solid. The invention can be Q-switched using techniques known in the art. In particular a thin film of slid dye switch placed within the index matched material can accomplish such switching of the invention. Experiments using the invention have also shown that its output laser beam can be modulated by modulating the excitation light.